1. Field of the Invention
The present invention relates to an internal combustion engine, and more particularly to a spark timing control device for an internal combustion engine.
2. Description of the Prior Art
Conventionally, in the design of an internal combustion engine mounted on an automotive vehicle, various considerations have been made from the standpoint of improving the performance of the engine. For example, in setting the spark timing, a manifold vacuum advance angle mechanism or a centrifugal advance angle mechanism has been adopted so as to obtain an optimal fuel consumption and a maximum output: a spark timing at a point very close to so-called M.B.T. (Minimum advance for Best Torque). On the other hand, in the relatively low speed range of a conventional spark ignition internal combustion engine, the critical spark time point beyond which knocking occurs and which depends on the kind of the engine and the octane number of the fuel used is on the side retarded from M.B.T., and therefore knocking may occur if the spark time point is set at M.B.T. Thus, in order to set the spark time point as close as possible to M.B.T. within the scope in which no knocking occurs, and in a relatively low speed region of crankshaft rotation, it is most recommendable to set the spark time point at the critical spark time point. However, there are, in fact, the influences of the engine aging and inhomogeneities in the fuel octane number which affect operation, and therefore it is inevitable that the spark time point will be set on the side further retarded from the critical spark time point, thereby increasing the fuel consumption and lowering the power output. The fuel consumption increase and power output decrease are especially large in an engine which is supercharged or has a high compression ratio, both of which characteristics have recently been considered as a means for decreasing the fuel consumption and increasing the power output, which is a big problem with these engines.
Although the spark time point may be set with some leeway on the side retarded from the critical spark time point, there is a possibility of knocking occurring depending on the driving conditions of an automotive vehicle, the operating condition of the engine, and the environmental conditions. It has been impossible to avoid knocking completely under any conditions.
On the other hand, if the spark time point is set so retarded as to prevent any knocking, the fuel consumption will be greatly increased and the power output will be greatly lowered, thereby bringing about no practical advantage.
Thus, conventionally, the spark timing has been controlled by a manifold vacuum advance angle mechanism or a centrifugal advance angle mechanism which have conventional or slightly advanced spark timing characteristics. Only when knocking actually occurs, the spark timing has been slightly retarded in order to avoid further knocking, as disclosed in U.S. Pat. No. 4,002,155.
A spark timing control device of this type starts to operate by detecting knocking occurring in the engine. A system for detecting this knocking according to the U.S. Patent mentioned above counts the number of portions of a signal containing knocking frequency components from an engine vibration sensor which exceed a reference level, and determines knocking according to the number of counted portions. Therefore, this determination is liable to be affected by noise, so that the signal containing knocking frequency components must be compared with another higher reference level in order to positively determine knocking. Thus the device cannot avoid use of complicated structure and often fails to operate reliably.
Such a spark timing control device carries out sampling for each predetermined period to determine the presence of knocking, retards the spark timing by a predetermined angle each time knocking occurs, and advances the spark timing by another amount when no knocking occurs. However, according to this structure, the control of retarding and advancing the spark timing by a predetermined amount is not carried out until completing the sampling. Therefore, the control response is slow. Further, since the angle by which the ignition is retarded is constant and independent of the frequency of the knocking, this retarded angle does not necessarily correspond to the required amount, and a retarded angle which eliminates the knocking is not obtained unless a considerable number of samplings are carried out, thereby not avoiding undesirable response.